Spiral wound thin film composite membrane elements are frequently used in reverse osmosis, nanofiltration, ultrafiltration, and microfiltration systems for the purification of tap water, seawater and brackish water, ultra-pure water production, wastewater treatments, and separation of organics from aqueous streams. The membrane of a spiral wound element is usually formed by attaching a flat sheet to a backing material that is typically a non-woven material. The filter membrane (i.e., the flat sheet and the backing material) is then folded around a feed spacer material, typically a polypropylene web, to form an envelope so that the backing material forms the outer surface of the envelope. The backing material is then adhered to a fabric sheet, such as tricot, with an adhesive, or sealant, such as glue. These layers are then wrapped around a permeate outlet tube. Typically, a spiral wound element will have several layers of tricot with a corresponding layer of filter membrane envelope in between.
The purpose of the sealant or adhesive, e.g., glue, is to prevent unfiltered feed fluid from entering the fabric sheet directly through the cross-section of the tricot or of the backing material. Unfiltered feed fluid that enters a filter membrane through the cross-sectional area may become trapped within the membrane or may work its way into the permeate side of the element, such as through the backing material into the fabric material, transporting the permeate to the permeate outlet tube. When the trapped unfiltered feed fluid is a liquid containing dissolved gases, the dissolved gases may come out of solution and expand when a pressure drop occurs, such as at the end of a filtration cycle. This can result in blisters or delamination of the element. Such blisters generally appear on the membrane over the glue line at either end of the element. As filtration continues, more unfiltered feed fluid is fed to the blister, causing further separation of the membrane from the backing until the blister breaks and creates a bypass path through which unfiltered feed fluid may enter the permeate outlet.
Trapped unfiltered feed fluid also can act as a source of permeate fluid contamination, such as by enhancing the growth of bacterial contaminants. Due to the location of the blisters outside of the normal flow path, they cannot be easily sanitized by forcing cleaning agents through the filtration system. The glue line also can be ruptured or damaged by telescoping, an excessive pressure drop from feed to concentrate, or by tearing of the membrane, such as when the permeate pressure exceeds the feed/concentrate pressure by a certain amount at any time. Spiral wound elements can be damaged at glue lines if the pressure on the filtrate side of the membrane exceeds that on the feed side. Factory imperfections such as glue line gaps or potting defects may cause integrity breaches as well.
To prevent blistering in spiral wound elements of thin film composite membranes, techniques have been used that include applying a sealant to the upstream or feed input surface of the membrane material where the potential for blistering exists (see, e.g., U.S. Pat. Pub. No. 2005/0121380) and sealing the pores of the filter membrane with a polymer to prevent an undesired fluid flow path through the filter element (see, e.g., U.S. Pat. Pub. No. 2003/0034293). Although these techniques address the problem of blister formation along the glue lines, they do not increase the glue penetration between the membrane layers. It has been found that increased glue penetration leads to elimination of blisters on the glue lines, consistency of glue line adhesion, and minimization of active area membrane damage.
Thus, there remains a need to develop filtration membranes and elements, including spiral wound reverse osmosis, nanofiltration, ultrafiltration, and microfiltration membranes and elements, that remain free from blisters on the glue lines when used in the purification of water. Also desired are methods for increasing glue penetration through the membrane layers. Accordingly, it is among the objects herein to provide an apparatus and method for applying pressure to the glue lines of spiral wound membranes and elements that prevent or eliminate blisters along the glue lines and minimize damage to the active area of the membrane.